The present invention relates to a loop type standard magnetic field generator providing a constant ratio of magnetic field strength to release voltage independent of signal frequency over a wide frequency range covering both low and high frequencies.
It has been known that corrections of indicated values of measuring devices such as electric field intensity measuring devices or interference wave measuring devices, or sensitivity tests of devices such as radio frequency receivers, all of which utilize loop antennas or bar antennas, are generally performed by using a standard magnetic field generated by a loop type standard magnetic field generator.
One typical example of a conventional loop type standard magnetic field generator for this purpose comprises a standard loop antenna constituted of a loop within which a transmission line having an outer conductor and an inner conductor electrostatically shielded by the outer conductor is coaxially provided and a terminal resistor through which one end of the inner conductor is connected to the loop, and an oscillator for feeding a signal to the loop antenna via the transmission line.
In the loop type standard magnetic field generator constructed in this way, the magnetic field strength at a given point in the magnetic field generated by the standard loop antenna thereof is determined by the release voltage of the oscillator, the configuration of the standard loop antenna and the distance between the loop antenna and said point. It has been known that although the ratio of the magnetic field strength of the standard magnetic field to the release voltage is substantially constant for the relatively low frequency range, it varies with change of frequency in the higher frequency range due to the transmission characteristics of the space between the standard loop and the point of measurement, the self-inductance of the loop and the stray capacitance existing between the inner conductor and the shielding conductor of the loop, etc.
For this reason, the conventional loop type standard magnetic field generator has an inherent disadvantage that the measuring accuracy in the high frequency band is degraded. Although this degradation may be overcome in various ways, it is impossible to avoid sophisticated testing and correcting procedures in doing so.
A primary object of the present invention is to provide a loop type wide band standard magnetic field generator providing a substantially constant ratio of magnetic field strength to indicated value of an indicator independently of the output frequency of the signal oscillator.